1 INTRODUCTION

We very much appreciate the comments of André Pestilho and Agustin Mors on our paper discussing the early diagenesis of the Pre-salt carbonates (Barra Velha Formation, BVF) in the Santos Basin (Gomes et al., 2024). We welcome the opportunity to respond and to clarify our interpretations concerning dolomite formation, diagenetic timing and isotopic signatures supported by additional examples.

2 SADDLE DOLOMITE

We agree that image D in our Figure 4 (that is figure 1A in Pestilho & Mors, 2025) and our image H in Figure 4 (figure 1B in Pestilho & Mors, 2025) in our paper (Gomes et al., 2024) do not show clear curved cleavages nor distinct curved crystal boundaries, which are typical of saddle dolomite. The presence of undulose extinction was the main criterion used in this study to identify saddle dolomite. However, other images, which we did not include in Gomes et al. (2024) for reasons of writing a concise paper, do show these features, along with the presence of subcrystals, fibres and domains revealed under crossed polars (see Figure 1A–F in this Reply). Thus, we are confident that this dolomite is of the saddle type as described by Warren (2000) and many others. Undulose extinction in saddle dolomite overgrowths around rhomb dolomite (commonly ‘cloudy’) is also a common feature of saddle dolomite in general, as shown in figure 4H of Gomes et al. (2024), also presented as figure 1B of Pestilho and Mors (2025). Our interpretation has been consistent for this mineral phase in all thin sections. This early diagenetic saddle dolomite phase has been recognised in 384 of the 2102 samples examined, with sizes generally around 100 μm but usually in low abundance. Our best pictures of this saddle dolomite are figure 4B, C, E, F, H and I in Gomes et al. (2024), along with Figure 1A–F in this Reply.

With regard to the comment of Pestilho and Mors (2025): ‘in the supplementary materials the authors have mentioned post-compactional SD, but they did not present any pictures’, we interpret these similar ‘saddle dolomite’ or ‘carbonate overgrowth’ crystals as having formed at two different times: during early diagenesis and during later diagenesis (see figure 11A in Gomes et al., 2024).

The saddle dolomite phase identified and interpreted as early diagenetic, in some cases forming around pre-existing rhombohedral or subhedral crystals, shows undulating extinction and subcrystals. Generally, this dolomite does not form large crystals, which makes it challenging to identify all the typical characteristics of saddle dolomite. Nevertheless, our classification of this phase was based on the observations of over 2000 thin sections. In view of its petrographic and cathodoluminescence features, we believe this carbonate form is best described with the term saddle dolomite.

3 TIMING OF DIAGENETIC EVENTS

As Pestilho and Mors (2025) point out, we did not describe the late-stage burial dolomitisation and the precipitation of saddle dolomite as a result of late hydrothermal activity. Whilst burial diagenesis is of undoubted interest, it is tangential to the stated objectives of our paper, which are to answer the questions: (1) What are the relationships between facies, mineralogy and texture, and the δ¹⁸O and δ¹³C values of the main constituents of the BVF? (2) How can isotopes contribute to understanding Pre-salt BVF paragenesis and the order of early diagenetic events? (3) How has diagenesis affected and altered depositional signals? The precipitation of saddle dolomite as crystals and overgrowths on rhombs, together with ‘shrub/spherulite dissolution’ and the presence of ‘macrocrystalline silica’ and ‘replacive silica’, records both early (syn-depositional and pre-burial) alteration, attributed to shallow discharge of high temperature hydrothermal fluids, and later burial diagenesis (see figure 11A in Gomes et al., 2024). These diagenetic processes have been reported in other areas of the Santos (Carvalho et al., 2022) and Campos basins (Lima & De Ros, 2019; Lima et al., 2020). It is of note that, at least in our study area, quantitative data show the low potential of these processes to alter depositional isotopic signals.

Carbonate rocks, as is widely known, are highly susceptible to diagenetic changes, which can occur rapidly, even within a time frame of months or years. The carbonate deposits of the BVF, though many millions of years old, have experienced diagenetic overprinting from multiple events, making their full characterisation a complex procedure. Nevertheless, certain diagenetic features are consistently observed across the BVF throughout the Santos Basin and within its time-equivalent in the Campos and Kwanza basins, whereas other diagenetic features are more localised.

4 HYDROTHERMAL EVENTS AND FLUID INCLUSIONS

With regard to the matter of hydrothermal fluids and fluid inclusions raised by Pestilho and Mors, hydrothermal events have been documented within the BVF, both during its deposition (as in the formation of travertine deposits, as noted by Falcão 2015) and Fontaneta et al. (2024) and long after during its burial. These events are linked to both deep fault systems that provide permeable conduits extending to depth (Carvalho et al., 2022; Correa & Carvalho, 2024) and later diagenetic processes (Carramal et al., 2022; Leite et al., 2020; Lima et al., 2020), as well as to locally enhanced heating, and perhaps fluids, derived from the intrusion of igneous rocks (Oliveira et al., 2024). Identifying and characterising the most significant hydrothermal events in the basin remain challenging, but it is critical for the geological modelling of reservoirs. This issue will be addressed in future studies on the same area, and fluid inclusions will be analysed to provide data on the timing of the hydrothermal activity: early and/or late diagenetic. It will then be possible to compare with areas studied by the other referenced authors.

In the wells studied, we observed at least two phases of saddle dolomite precipitation (see figure 11A; Gomes et al., 2024). The interpretation that the first phase occurred during eodiagenesis is based on both the petrographic features (Figure 1 here) and the position of these features within stratigraphic cycles. Syn-depositional oxidation of organic matter at shallow burial depths likely facilitated the dissolution of magnesian clay and generated Mg-rich pore fluids. Additionally, some cycles are linked to intervals of subaerial exposure, during which sediments would have interacted with meteoric water which would have been undersaturated with respect to magnesian clay. In the paragenetic evolution, we identified that the dissolution of magnesian clay occurred simultaneously with the first phase of saddle dolomite formation. Locally, hydrothermal vents may have contributed to the formation of this early saddle dolomite. However, on a larger scale, the predominant diagenetic feature was the dissolution of magnesian clay. We acknowledge that the more negative oxygen isotope values could be due to either meteoric water or to the venting of a hydrothermal fluid. Since we did not perform fluid inclusion analyses to distinguish between these two possibilities, we stated in section 5.7 of Gomes et al. (2024), paragenesis stage 3, that the results ‘suggest the potential influence of meteoric water or hydrothermal vents’. It was not within the scope of this study to conduct fluid inclusion analyses, so both possibilities were left open.

5 U–PB AGES OF MINERAL PHASES

We do agree with Pestilho and Mors (2025) that U–Pb ages would be very useful in future studies from our area, specifically from saddle dolomite, to test the hypotheses for its origin. The discussion presented here aims to contribute to the understanding of this complex process. The information provided does not negate or contradict previous studies; rather, it adds a layer of complexity by documenting the influence of hydrothermal processes during the early phases of diagenesis.

We would like to clarify that the objectives of our article were to emphasise the early diagenetic processes because, from our quantitative evaluation, they seem to have been more volumetrically important in our area of study. We identified a gap in the literature regarding the influence of hydrothermal activity during or soon after deposition. This may involve deep-sourced fluids and/or convection of waters sourced from the lake, as previously modelled for a marine system by Benjakul et al. (2020). However, this does not discount or preclude later hydrothermal events within the basin, as included in our paragenetic sequence (figure 11A, Gomes et al., 2024). Our aim in this study was not to address all possible diagenetic processes that may have occurred in the basin but to focus specifically on the early stages of diagenesis, including the influence of hydrothermal fluids during and soon after deposition.

6 INTERPRETATION OF TRENDS IN Δ¹³C AND Δ¹⁸O

We agree that δ¹⁸O signatures in early cements might record the influence of later diagenesis. Although Lima et al. (2020) showed greater negative δ¹⁸O shifts for saddle dolomite, it does appear that Lima et al. (2020) did not use secondary ion mass spectrometry to provide the isotope analyses of single mineral phases. For δ¹³C and δ¹⁸O, they analysed 51 samples, of which 48 were bulk rock where more than 90% of the samples had a single carbonate phase and 21 samples were obtained using a micro-mill to provide powder for specific mineral analyses. All those samples were submitted to a traditional isotope ratio mass spectrometer (IRMS). Our work used a similar method (IRMS) to analyse powders from both bulk rock and micro-drilling: ‘For whole-rock analysis, around 30 μg of powder was collected from 947 sidewall rock samples and plugs from cores. From 52 thin sections, each 120 μm thick, 490 powders were extracted by micro-drilling individual spherulites and shrubs and crystals of rhombohedral, lamellar, anhedral and saddle dolomite’. We should have mentioned that we used a computer-monitored New Wave Research MicroMill TM instrument. The same instrument was used by Lima et al. (2020). The author L. R. Tedeschi participated in both papers and can testify to the methods. The late diagenetic mineral results presented by Lima et al. (2020) from the Campos Basin show a quite different oxygen isotope composition (many values between −4 and −10‰) compared to all samples analysed in our area of study (typically less negative than −2‰, see our figure 12 (in Gomes et al., 2024), for which most of the data were micro-drilled). Although we cannot rule out that carbonate mineral phases with δ¹⁸O values less negative than −2‰ are not related to late diagenesis, it is more likely that they represent mineral phases formed during early diagenesis. These values are similar to oxygen isotope data from early hydrothermal activity (>−4.5‰) which occurred during the deposition of the BVF, as demonstrated by the reported travertine facies in other areas of the Santos Basin (Fontaneta et al., 2024).

7 RECRYSTALLISATION

Regarding the recrystallisation of shrubs and spherulites, we did observe their occurrence, which is why we included this in the Supplementary Material due to the volume of topics and data already covered in our paper. Since the objectives outlined earlier focussed on early diagenetic processes and alterations, we did not provide the necessary data to determine the timing of calcite recrystallisation, which may have occurred at various stages. In our treatment of the isotopic results, we considered the alteration of these minerals by dolomitisation to be more significant. Consequently, we placed greater emphasis on the degree of dolomitisation in figures 6 and 7 (in Gomes et al., 2024) for the interpretation of the isotope data, rather than on the recrystallisation of calcite.

Geochemical analyses are invaluable tools for understanding and interpreting diagenetic environments, but they can also be influenced by the superimposition of events. In this study, petrographic relationships served as the primary analysis tool, where cementation and dissolution events were positioned relative to one another over time, using mechanical and chemical compaction features as a guide. Geochemical analyses were treated as complementary tools to help interpret the chemical conditions associated with each diagenetic phase. Additional studies, such as absolute dating and fluid inclusion analysis, are currently underway and may provide further clarity on the questions left open in this work. Anyway, it is important to highlight that all spherulites and shrubs with or without recrystallisation have δ¹⁸O values typically less negative than −2‰ in our studied area. Therefore, recrystallisation is unlikely to have occurred at high temperatures, such as those deduced for late diagenetic processes by Lima et al. (2020). Nevertheless, future studies on fluid inclusions and strontium isotopes might shed light on doubts that may persist in the Pre-salt rocks from our area of study.

8 CONCLUDING REMARKS

In response to the comments of Pestilho and Mors (2025), we would like to confirm that our article is not a review paper but one focussing on petrography and stable isotope analysis of the early diagenetic events in a specific area of the Santos Basin with results and interpretations that are different from the previously published studies mentioned above. Although we referenced works by authors such as Falcão (2015), Fontaneta et al. (2024), Carvalho et al. (2022), Correa and Carvalho (2024), Carramal et al. (2022), Lima et al. (2020), Leite et al. (2020) and Oliveira et al. (2024), our intention was not to offer a comprehensive review of all diagenetic processes across the basin for all facies of the BVF. Each of the cited works drew from field data collected at different locations within the basin, offering valuable insights specific to their study areas. There is now a vast body of literature addressing these Aptian deposits along the southern margin of the Atlantic. If Pestilho and Mors feel it is essential and timely to undertake a review that includes all publications and perspectives on the subject, we encourage them to conduct this analysis and make comparisons.

Future studies using new techniques may provide further insights, and we support these efforts as long as they are appropriately framed within the correct phase of diagenetic evolution for each area. Finally, we believe that both regional features and local effects must be integrated for a comprehensive understanding of the basin in future studies of the Pre-salt.